1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method and image processing program for determining such attributes as screen halftone area, character area and continuous tone area that appear in image data.
2. Description of the Related Art
In recent years, the digital copying machine has been spreading, replacing the analog copying machine. The digital copying machine scans an image on a document by charge coupled device (CCD) sensor etc. and converts a signal of the image into digital data. Furthermore, the digital copying machine performs such image processing as filtering, gamma-correction, half-toning etc. for the obtained data of the image and prints the image on paper by laser printer etc.
On the image of the document, there appear various areas such as screen halftone area, character-line drawing area and continuous tone area. Modes of image processing to record on paper an image in high picture quality depend on which area the image or part of the image belongs to. To meet the needs of picture quality, there are some types of the digital copying machines that determine attribute to see which area the image data belong, separate the data according to the attribute and differentiates modes of image processing according to the separated data.
Various separation techniques on the basis of an image data expressed in the space domain have been disclosed. Even in case data expressed in a spatial frequency domain as a result of orthogonal transform such as fast Fourier transform (FFT) and discrete cosine transform (DCT) are handled, the data may be inversely transformed in order to apply the prior art technique. In this case, after separation is carried out on the basis of image data expressed in space domain as the result of inverse transform, orthogonal transform is carried out.
Meanwhile, there are also techniques in which separation is conducted on the basis of data expressed in the spatial frequency domain. Unexamined Japanese patent application No. 6-223172 discloses a technique of separating the character area and non-character area on the basis of data obtained by performing wavelet transform on image data. Also unexamined Japanese patent application No. 11-220622 describes a technique in which an attribute is determined for every object block on the basis of data obtained by performing wavelet transform on image data and each block is separated into character area, screen halftone area or continuous tone area on the basis of the attribute.
However, in case the technique is adopted which does separation on the basis of image data expressed in the space domain, inverse transform and orthogonal transform have to be done for separation, taking extra processing time, when data expressed in the spatial frequency domain is handled.
Also, even in case the technique is adopted of doing separation on the basis of data expressed in the spatial frequency domain, it can happen that the picture quality is insufficient if an image is merely separated into the character area and the non-character area as in a technique described in unexamined Japanese patent application No. 6-223172. Furthermore, even if an image is separated into the character area, screen halftone area and continuous tone area on the basis of the wavelet transformed data as in the technique described in unexamined Japanese patent application No. 11-220622, since the separation is done block by block, error of determination could have a great adverse effect, decreasing the picture quality.
As set forth above, in such image processing apparatuses like the digital copying machine and scanner, the attributes of image data have to be determined in detail and a mode of image processing suitable for the attribute has to be carried out, but no sufficient attribute determination can be done.